Fuel cells generating electricity by an electrochemical reaction of hydrogen (fuel gas) with oxygen (oxidizing gas) have been noted as the effective energy source. One typical example of the fuel cell has a stack structure of multiple cells, where each cell has a membrane electrode assembly that is obtained by attaching an anode (hydrogen electrode) and a cathode (oxygen electrode) to respective surfaces of a proton-conductive electrolyte membrane and is interposed between a pair of separators. Hereafter the fuel cell having this stack structure is also called the fuel cell stack.
The fuel cell stack generally has supply manifolds (a fuel gas supply manifold and an oxidizing gas supply manifold) for distributing the supplies of reactive gases (a fuel gas and an oxidizing gas) to the anodes and the cathodes of the respective cells and exhaust manifolds (an anode off gas exhaust manifold and a cathode off gas exhaust manifold) for collecting the flows of an anode off gas and a cathode off gas from the anodes and from the cathodes of the respective cells and discharging the collected flows of the anode off gas and the cathode off gas out of the fuel cell stack.
One proposed structure of the fuel cell stack recirculates the anode off gas containing unconsumed fuel gas by power generation to the anodes of the respective cells for the effective use of the fuel gas. Another proposed structure of the fuel cell stack utilizes the fuel gas supplied to the anodes of the respective cells for power generation without discharge of the anode off gas out of the fuel cell stack or recirculation of the anode off gas to the anodes of the respective cells. The latter structure is called the anode dead end-type fuel cell.
In the fuel cells of any structures, an impurity gas that is included in the fuel gas and has no contribution to power generation is accumulated on the anodes of the respective cells. When the air is used as the oxidizing gas, an impurity gas, such as nitrogen, which is included in the air supplied to the cathodes and has no contribution to power generation, is transmitted through the electrolyte membranes and is accumulated on the anodes of the respective cells. Accumulation of the impurity gas on the anodes lowers the relative concentration of the fuel gas, thus decreasing the power generation performance of the fuel cell and deteriorating the membrane electrode assemblies. Deterioration of the membrane electrode assemblies is mainly ascribed to oxidation of carbon included in the cathodes. This problem is especially noticeable in the anode dead end-type fuel cells, where the fuel gas is accumulated on the anodes during power generation. Several techniques have been proposed to intermittently discharge the impurity gas-containing anode off gas accumulated on the anodes out of the fuel cell (see, for example, Japanese Patent Laid-Open Gazette No. 2005-166498, No. 2004-327360, and No. 2005-243477).
The control technique disclosed in Japanese Patent Laid-Open Gazette No. 2005-166498 measures a local current in a specific site in each cell having a high potential for deficiency of hydrogen (for example, in the vicinity of a hydrogen outlet in each cell) in the fuel cell and discharges the impurity gas-containing anode off gas out of the fuel cell in response to the local current of lower than a preset reference current value, in order to increase the hydrogen concentration in the cell. In a fuel cell stack of several hundred cells having an identical internal structure, it is not impossible but is highly impractical to measure the local current in all the cells for detection of the hydrogen deficiency in the respective cells. One available method measures the local current in only part of the cells among the several hundred cells for detection of the hydrogen deficiency. There is, however, a certain possibility that other cells as non-target of local current measurement have hydrogen deficiency, even when no hydrogen deficiency is detected in the part of the cells selected as the target of local current measurement.
The control technique disclosed in Japanese Patent Laid-Open Gazette No. 2004-327360 discharges the impurity gas-containing anode off gas of the respective cells out of the fuel cell, in response to a decrease in concentration of the fuel gas flowing in the whole fuel cell to or below a preset reference level or in response to an increase in concentration of the impurity gas to or above a predetermined reference level. The technique disclosed in Japanese Patent Laid-Open Gazette No. 2005-243477 collectively stores the anode off gas from the respective cells in an external buffer provided outside the fuel cell and discharges the stored anode off gas from the external buffer in response to a decrease in concentration of the fuel gas included in the anode off gas stored in the buffer to or below a preset reference level. There is inevitably a manufacturing variation in pressure loss among fuel gas flow paths of the respective cells. There is also a variation in decrease of the fuel gas concentration in the respective cells. These proposed techniques can detect only the overall decrease of the fuel gas concentration or the overall increase of the impurity gas concentration in the whole fuel cell stack, despite such variations. A relatively high discharge frequency of the fuel gas-containing anode off gas out of the fuel cell is required to avoid the potential problem in the individual cells. Namely the fuel gas usable for power generation is wastefully discharged out of the fuel cell. There is accordingly still room for improvement in the effective use of the fuel gas in the fuel cells stack.